Recently, magnetic particles have been proposed as means for collecting a target substance efficiently. Since magnetic particles can be collected easily and efficiently by using an external magnetic field, they are used as accurate detection means in methods for detecting biological substances and the like. In order to surely collect a target substance of very small size in a microenvironment, such as a virus molecule in an organism, the surface area of the nanoparticles for collection has to be large. Therefore, vaccines and methods for separating, concentrating, or detecting viruses have been proposed which use small-sized nanoparticles (See, for example, “Biomaterial-Seitaizairyou” vol. 22, no. 6 (2004), pp. 394-399). Japanese Patent Application Laid-Open (JP-A) No. 2002-165591 discloses adsorption of viruses by magnetic particles which have lectin on its surface and which consist of polymer particles containing a magnetic material in their interior portions.
However, when the particle diameter of the magnetic particles to be used is large, the amount of the adsorbed target substance and the analysis sensitivity are insufficient though the magnetic particles show better response to magnet. In contrast, when the particle diameter is not larger than several tens of nanometers, the magnetic particles show inferior response to magnet; therefore accurate analysis is difficult.
In order to provide magnetic nanoparticles of nano-class which can surely respond to an external magnetic field, it has been proposed to aggregate the magnetic nanoparticles by using a polymer having a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST) (See, for example, International Publication No. 02/16571 pamphlet and JP-A No. 2002-60436).